1. Field of the Invention
The present invention relates to a method and an apparatus for cleaning a substrate by scrubbing a surface to be cleaned of the substrate while the substrate is in rotation with a roll-shaped cleaning member, such as a roll brush, a roll sponge, or the like, which is being held in contact with the surface to be cleaned of the substrate.
2. Description of the Related Art
As semiconductor devices are finding themselves integrated to increasingly higher levels, there is a demand in the art for the development of a cleaning technology for highly cleaning an entire surface (front surface and/or back surface) of substrates in order to achieve a high yield of product substrates. For example, CMP (Chemical Mechanical Polishing) processes, which are performed to planarize insulating films, produce STIs (Shallow Trench Isolations), form tungsten plugs, and form multilayer copper interconnects, generally employ a contact-type scrubbing process for effectively removing remaining residue from polished substrate surfaces. When substrates having devices of smaller dimensions are polished, metal interconnects that are exposed by the polishing process may possibly be chemically or electrochemically corroded under etching forces of chemicals used and mechanical forces. Such metal interconnect corrosions are considered to adversely affect the reliability of the devices significantly. Accordingly, it has been desired in the art to develop a suitable cleaning technology which is capable of effectively removing remaining residue from polished substrate surfaces while minimizing any adverse effects to devices.
Generally, CMP apparatuses are configured to operate by scrubbing a surface of a substrate, such as a semiconductor wafer or the like, with a roll-shaped cleaning member, such as a roll brush, a roll sponge, or the like, while the roll-shaped cleaning member and the substrate are being rotated about their own axes and also while the roll-shaped cleaning member is being held in contact with the surface of the substrate under a predetermined pressure. It is customary in the scrubbing process to place the roll-shaped cleaning member in a position where the central axis of the roll-shaped cleaning member and the central axis of the substrate, i.e., the central axis, about which the substrate rotates, cross each other perpendicularly (see Japanese patent No. 4023907, patent documents 1, and Japanese patent No. 3854085, patent document 2).
When the surface (surface to be cleaned) of the substrate is scrubbed by the roll-shaped cleaning member that is placed in the above position, however, the roll-shaped cleaning member contacts the surface of the substrate with a higher contact density in a central area of the substrate and with a lower contact density in a peripheral area of the substrate. As a result, the cleaning intensity over the entire surface of the substrate suffers irregularities due to the different contact densities, making it difficult for the scrubbing process to meet desired cleaning requirements. The failure to meet the desired cleaning requirements has a detrimental effect on efforts to prevent, e.g., local interconnect corrosions on the surface of the substrate.
FIGS. 1 and 2 schematically show a conventional substrate cleaning apparatus which uses roll brushes as roll-shaped cleaning members. As shown in FIGS. 1 and 2, this substrate cleaning apparatus includes an upper roll brush (roll-shaped cleaning member) 10 disposed in contact with a front surface (upper surface) of a substrate W for scrubbing the front surface of the substrate W, and an upper cleaning liquid supply nozzle 12 for supplying a cleaning liquid to the front surface of the substrate W. The upper roll brush 10 and the upper cleaning liquid supply nozzle 12 are positioned upwardly of the substrate W. This substrate cleaning apparatus also includes a lower roll brush (roll-shaped cleaning member) 14 disposed in contact with a back surface (lower surface) of the substrate W for scrubbing the back surface of the substrate W, and a lower cleaning liquid supply nozzle 16 for supplying a cleaning liquid to the back surface of the substrate W. The lower roll brush 14 and the lower cleaning liquid supply nozzle 16 are positioned downwardly of the substrate W.
As shown in detail in FIG. 3, each of the upper roll brush 10 and the lower roll brush 14 comprises a roll brush 18 having a number of cylindrical nodules (protrusions) 18a on its outer circumferential surface. The nodules 18a have projecting distal end faces held in contact with the surface to be cleaned of the substrate W across a contact width Li.
The upper roll brush 10 is placed in a position where the central axis O1 thereof and the central axis O2 of the substrate W, i.e., the central axis, about which the substrate W rotates, cross each other perpendicularly. While the cleaning liquid is being supplied from the upper cleaning liquid supply nozzle 12 onto the front surface of the substrate W, the upper roll brush 10 is pressed against the front surface of the substrate W under a predetermined pressure. At the same time, the upper roll brush 10 is rotated about the central axis O1 and the substrate W is rotated about the central axis O2, thereby scrubbing the front surface of the substrate W.
Similarly, the lower roll brush 14 is placed in a position where the central axis O3 thereof and the central axis O2 of the substrate W cross each other perpendicularly. While the cleaning liquid is being supplied from the lower cleaning liquid supply nozzle 16 onto the back surface of the substrate W, the lower roll brush 14 is pressed against the back surface of the substrate W under a predetermined pressure. At the same time, the lower roll brush 14 is rotated about the central axis O3 and the substrate W is rotated about the central axis O2, thereby scrubbing the back surface of the substrate W.
Thus, the front and back surfaces of the substrate W are cleaned under identical conditions. Therefore, the process of cleaning the front surface of the substrate W with the upper roll brush 10 in the form of the roll brush 18 shown in FIG. 3 will be described below.
When the nodules 18a of the roll brush 18 pass over each point on the front surface of the substrate W, the distance that the front surface of the substrate W is rubbed by the nodules 18a per unit time is calculated. It is assumed that an average value of the calculated distances at positions at a radius r along a circumferential direction on the front surface of the substrate W is referred to as a cleaning intensity Rc (m/s). It is believed that the uniformity with which the entire front surface of the substrate W is cleaned can be evaluated by the cleaning intensity Rc (m/s). The cleaning intensity Rc (m/s) at the radius r on the front surface of the substrate W is expressed by the following equation:
      R    c    =            nLi              R        r              ⁢          V      rw      where n represents the number of nodules 18a formed on the roll brush 18 along the circumferential direction of the roll brush 18, Li the contact width across which each nodule 18a contacts the front surface of the substrate W, i.e., the diameter of the nodule 18a, as shown in FIG. 3, Rr the radius of the roll brush 18, and Vrw the average value of relative speeds between the distance end face of the nodule 18a and the front surface of the substrate W along the circumferential direction of the substrate W. The average value Vrw is expressed by the following equation:
      V    rw    =            1              2        ⁢        π        ⁢                                  ⁢        r              ⁢                  ∫        0                  2          ⁢          π                    ⁢                                                            V              _                                      rw              ⁡                              (                θ                )                                                              ⁢        r        ⁢                                  ⁢                  ⅆ          θ                    where Vrw(θ) represents each position along the circumferential direction. Each position Vrw(θ) along the circumferential direction is determined by the following equation:
            V      _              rw      ⁡              (        θ        )              =      {                                        0            ⁢                                                  ⁢            Noncontact            ⁢                                                  ⁢            area            ⁢                                                  ⁢            between            ⁢                                                  ⁢            nodule            ⁢                                                  ⁢            and            ⁢                                                  ⁢            substrate                                                                                          V                _                            r                        -                                                            V                  _                                w                            ⁢                                                          ⁢              Contact              ⁢                                                                                ⁢                                                                              ⁢              area              ⁢                                                          ⁢              between              ⁢                                                          ⁢              nodule              ⁢                                                          ⁢              and              ⁢                                                          ⁢              substrate                                          where Vr represents the vector of the speed of the end face of the nodule 18a, and Vw the vector of the speed of the front surface (surface to be cleaned) of the substrate W.
FIG. 4 shows the relationship between the cleaning intensity Rc and the radius r of the substrate W when the front surface of the substrate W is cleaned by the roll brush 18 at the time the roll brush 18 as the upper roll brush 10 rotates at a constant rotational speed of 100 rpm and the substrate W rotates at different rotational speeds of 50 rpm, 100 rpm, and 200 rpm. It can be seen from FIG. 4 that under any cleaning conditions the cleaning intensity Rc has a peak in an area near the center of the substrate W. The peak has a value that is 6 to 30 times a flat value of the cleaning intensity Rc, i.e., a substantially constant value of the cleaning intensity Rc. It can also be seen that the area exhibiting the peak of the cleaning intensity Rc lies within a radius of 25 mm on the front surface of the substrate W. This suggests that if the front surface of the substrate W is cleaned to achieve the sufficient flat value of the cleaning intensity Rc, then the area within the radius of 25 mm on the front surface of the substrate W is intensively cleaned and interconnect corrosions are developed.
FIG. 5 shows the calculated relationship between the cleaning intensity Rc and the radius r of the substrate W when the substrate W is cleaned by roll brushes 18 with nodules 18a having diameters (contact widths: Li) of 3 mm, 6 mm, 10 mm, and 15 mm. It can be seen from FIG. 5 that under any cleaning conditions the area where the substrate W is intensively cleaned is present near the center of the substrate W and as the contact width Li becomes smaller, the peak of the cleaning intensity Rc also becomes smaller in height and width. Table 1, below, shows the relationship between the radial position r of the substrate W and the contact width Li when the cleaning intensity Rc is 20% higher than the flat value thereof. It can be understood from Table 1 that in a practical range of contact widths, i.e., from 3 mm to 10 mm, the ratio (r/Li) of the radial position r of the substrate W to the contact width Li at the time the cleaning intensity Rc is 20% higher than the flat value thereof is about 8 or smaller.
TABLE 1Contact width Li (mm)361015Radius (mm) at 120% of flat value25252530
In order to prevent the entire front surface of the substrate from being cleaned irregularly by the roll-shaped cleaning member, which may be a roll brush, a roll sponge, or the like, it has been proposed to improve the shape of a roll brush by, for example, providing protrusions or nodules having a density or area which differs in the longitudinal direction of the roll brush on the outer circumferential surface thereof (see Japanese laid-open patent publication No. 2001-358110, patent document 3) or changing the outside diameter of a roll brush (see U.S. Pat. No. 7,185,384, patent document 4). However, since the roll brush is mounted on a polishing apparatus in a fixed positional relationship to the substrate according to any of the above proposals, it is conceivable that the distribution of cleaning intensities over the surface to be cleaned of the substrate cannot be adjusted as desired depending on the conditions under which the roll brush and the substrate rotate about their own axes.
There has been proposed a cleaning apparatus wherein a substrate moves back and forth in a horizontal plane parallel to a roll-shaped cleaning member as the substrate rotates about its own central axis (see Japanese laid-open patent publication No. 2000-77379, patent document 5). There has also been proposed a cleaning apparatus having an upper roll brush and a lower roll brush disposed such that axes of the upper roll brush and the lower roll brush extend parallel to each other and are biased perpendicularly to the central axis about which a substrate rotates (see Japanese patent No. 2887095, patent document 6).
The cleaning apparatuses disclosed in Patent documents 1 through 6, however, are not configured to take into account the cleaning intensity at each position (area) along the radial direction of the surface to be cleaned of the substrate in view of the contact width and contact frequency with which the substrate contacts the nodules. Consequently, even though the front surface (surface to be cleaned) of the substrate is cleaned by the roll-shaped cleaning member while the surface to be cleaned of the substrate is moving back and forth parallel to the roll-shaped cleaning member, or even though the upper roll brush and the lower roll brush are disposed such that axes of the upper roll brush and the lower roll brush extend parallel to each other and are biased perpendicularly to the central axis about which the substrate rotates, it is considered difficult to scrub the entire surface to be cleaned of the substrate with more uniform cleaning intensity.